The present invention relates generally to microturbine generator systems and the like. More particularly, the present invention relates to a power bridge topology and control techniques for a microturbine generator system.
Lower cost PWM inverter technology and electric utility deregulation have led to the demand for small ( less than 10 MW) turbo-electric generation machines. Sometimes referred to as microturbines, they are small, high-speed generator power plants that typically include a turbine, compressor, and generator, all of which are on a single shaft, as well as the power electronics to deliver the power to the grid. Microturbines typically have only one moving part, use air bearings and need no lubricating oil. These small power plants operate on natural gas, diesel, gasoline or other similar high-energy, fossil fuel.
It is generally desirable for a microturbine to include a method or components to start the turbine and a method or components to efficiently convert electrical power generated by the turbine into useful (e.g., 60/50 Hz) electricity.
Certain turbogenerator and motor control techniques are known. U.S. Pat. No. 5,903,116 discloses a turbogenerator/motor controller with a microprocessor-based inverter having multiple modes of operation. To start the turbine, the inverter connects to and supplies fixed current, variable voltage, variable frequency, AC power to a permanent-magnet (PM) turbogenerator/motor, driving the PM turbogenerator/motor as a motor to accelerate the gas turbine. Once self-sustaining operation is reached, the inverter is disconnected from the PM generator/motor, reconfigured to a controlled 60 Hertz mode, and then either supplies regulated 60 Hz three phase to a stand alone load or phase locks to the utility, or to other like controllers, to operate as a supplement to the utility. In this mode of operation, the power for the inverter is derived from the PM generator/motor via high frequency rectifier bridges. The microprocessor monitors turbine conditions and controls fuel flow to the gas turbine combustor.
U.S. Pat. No. 6,020,713 discloses a method of rectifying the output of a microturbine generator. A controller is provided for a permanent magnet turbogenerator/motor having a pulse width modulated inverter and a rectifier bridge. The PWM inverter includes a positive section DC bus and a separate equal negative section DC bus, and the inverter has a positive section connected to one magnetic winding of the PMG and a negative section connected to a separate magnetic winding of the PMG.
U.S. Pat. No. 6,023,135 discloses a method of controlling a permanent magnet turbogenerator/motor in which electrical power is provided to a permanent magnet turbogenerator/motor through a pulse width modulated inverter to start the permanent magnet turbogenerator/motor to achieve self-sustaining operation. Electrical power is disconnected from the pulse width modulated inverter once self-sustaining operation is achieved, and the pulse width modulated inverter is reconfigured to supply voltage from the permanent magnet turbogenerator/motor. In addition, and exhaust gas temperature from the permanent magnet turbogenerator/motor is maintained at a substantially constant value while supplying voltage.
U.S. Pat. No. 6,031,294 discloses a method of controlling a permanent magnet turbogenerator/motor in which utility electrical power is provided to the permanent magnet turbogenerator/motor through a pulse width modulated inverter to start the permanent magnet turbogenerator/motor to achieve self-sustaining operation, the utility electrical power is disconnected from the pulse width modulated inverter once self sustaining operation is achieved, the pulse width modulated inverter is reconfigured to supply voltage from the permanent magnet turbogenerator/motor, and an energy storage and discharge system is provided for the pulse width modulated inverter to provide electrical energy to the inverter when utility electrical power is unavailable to start the permanent magnet turbogenerator/motor and during self-sustained operation when the turbogenerator cannot meet an instantaneous load requirement and to otherwise store electrical energy during self-sustained operation.
Known turbogenerator and motor control techniques for microturbines such as those described above do not adequately address techniques for controlling DC bus voltage, which is important for optimal operation and control of the PWM inverter. They also do not provide for operation with AC Induction generators since they are unable to supply reactive currents to the generator.
Therefore, it would be desirable to provide a novel and unique technique for producing power from a microturbine which is highly efficient, cost-effective, and which requires relatively low maintenance and can work with either PMG or induction machines.
It would also be desirable for such a technique to specifically address the problem of controlling DC bus voltage in a microturbine and reduce the number of moving mechanical parts included in the system.
It would further be desirable for such a control arrangement to be able to maintain exhaust gas temperature while voltage is being provided, and to include an energy storage and discharge system for the pulse width modulated inverter to provide electrical energy to the inverter when utility electrical power is unavailable to start the permanent magnet turbogenerator/motor (or induction machine) and during self-sustained operation when the turbogenerator cannot meet an instantaneous load requirement and to otherwise store electrical energy during self-sustained operation.
The present invention overcomes the above-noted shortcomings of the prior art, and achieves additional advantages, by providing for a back to back PWM inverter topology and method for controlling its DC bus voltage. According to exemplary embodiments described below, a microturbine controller includes a permanent magnet generator connected to the turbine, a first converter connected to the permanent magnet generator, and a second converter connected to the first converter and to an electric utility interface. The bus voltage of a DC bus connected to the first and second converters is controlled by the second converter in a first mode of operation, and is controlled by the first converter in a second mode of operation. The second converter operates to control an output voltage or current in the second mode.
A microturbine controller in accordance with the present invention provides a reliable, efficient, and easily maintainable solution to start the microturbine, control a DC bus voltage associated with the microturbine, and generate power, and achieves additional advantages.